Bone cancers have grave clinical consequences and affect the lives of hundreds of thousands of patients in the United States each year. Novel strategies for treating these malignancies are desperately needed to advance treatment and embark on prevention of bone cancer. The pathophysiology of virtually all bone cancers is intimately linked to bone cancer-induced osteoclast formation. The long range goal of our laboratory is to understand the interaction between bone cancer and bone cells and to use this knowledge as the basis for developing novel methods for treating and preventing bone cancer. The objective of this proposal is to use knowledge of osteoclast (OCL) precursors and osteoclast lineage-specific gene promoters to develop an ex vivo gene delivery system that kills bone cancer cells. The central hypothesis of this proposal is that an osteoclast precursor-based, prodrug gene delivery system will permit killing of bone-residing cancers. The prodrug system selected is the cytosine deaminase/5 flourocytosine system. This hypothesis is based on Preliminary Studies, indicating that 1) OCL precursors home to sites of bone cancer, 2) OCL precursors can be purified and transduced to express selected genes, 3) customized retroviral vectors provide osteoclast lineage-specific expression of marker genes, and 4) a cytosine deaminase gene delivery system kills bone cancer cell in vitro. To evaluate the central hypothesis, the following aims will be pursued: 1. Demonstrate that transplanted osteoclast precursors will home to sites of bone cancer. 2. Identify the retroviral vector which achieves maximal and specific expression of a marker gene in osteoclast lineage cells. 3. Demonstrate that transplanted osteoclast precursors will express a maker gene specifically at sites of bone cancer. 4. Determine if an osteoclast precursor-mediated cytosine deaminase gene delivery system permits killing of bone cancer cells. Proving our central hypothesis will represent an extraordinary advance towards development of new cancer therapies and will shift the focus of bone cancer treatments from palliation to cure.